Field of the Invention
The invention relates to a micro feedback-chamber sensor and a method of manufacturing such sensor, and more particularly to a micro feedback-chamber sensor used in the acoustic wave and/or pressure and a method of manufacturing such sensor.
Description of the Related Art
Currently, most existing micro sensors are based on piezoelectric, piezoresistive and capacitive sensing principles. The piezoelectric sensor converts an input force signal into charges, which are accumulated across the piezoelectric material and can be outputted as a voltage. The piezoresistive sensor has the resistance change after the force is exerted on the piezoresistive material. The most frequently adopted sensing mechanism is based on the capacitive technology, and the capacitive sensor has the good advantages that it can be easily manufactured and that it has the high sensitivity and the low power consumption.
More particularly, a sensor, such as a micro pressure sensor and a micro microphone, has the feedback chamber design. For example, an absolute pressure sensor having a hermetic vacuum feedback chamber generates the structure deformation (piezoelectric, piezoresistive or capacitive physical amount) due to the pressure difference between the chamber and the outside. For instance, the micro microphone requires an acoustic feedback-chamber to reflect the received acoustic signal. Thus, one important sensor quality of the sensor having the feedback chamber design comes from the volume (size) of the feedback chamber. For example, the feedback chamber of the pressure sensor is configured to be kept in a fixed low-pressure state (i.e., approaching the high vacuum state) when being manufactured. After being manufactured, the outgassing effect of the walls of the feedback chamber gradually increases the gas pressure in the feedback chamber. The residual gas affects the measurement due to the thermal expansion and contraction principle. The ideal gas equation is known as pV=nRT, where p denotes the pressure of the ideal gas, V denotes the volume of the ideal gas, n denotes the amount of substance of gas, T denotes the thermodynamic temperature of the ideal gas, and R denotes the ideal gas constant. Therefore, if the designed volume of the feedback chamber is increased (V is equivalently increased), then the lower p can be obtained. So, the influence of the thermal expansion and contraction caused by the temperature effect on the sensor is lower.
In addition, the micro microphone is implemented using the conventional package technology, for example. FIG. 14 is a schematic view showing a conventional package of a micro microphone. As shown in FIG. 14, in the process of assembling a conventional micro microphone, a Micro-Electro-Mechanical-System (MEMS) sensing chip 520 and a signal processing chip 530 are separately mounted on a package substrate 510 and then electrically connected together by way of wire bonding. Then, a covering lid 540 covers the MEMS sensing chip 520 and the signal processing chip 530 to form a front chamber. The conventional packaging method can only handle the singulated chip, which way is time and material consuming, and is higher cost. When the package structure of the micro microphone is designed, the following important items have to be carefully evaluated and considered. First, the front chamber distance represents the distance of the space from the sound receiving port, from which the sound pressure enters the sensor, to the diaphragm of the sensor (the conventional distance is equal to the thickness of the covering lid 540 minus the thickness of the substrate 510 and that thickness of the MEMS sensing chip 520, and is generally greater than 300 microns). The too-long front-chamber distance increases the sound resistance and affects the quality. Therefore, in terms of design, the front-chamber distance should be as smaller as possible. Of course, the front-chamber distance may also correspond to the front-chamber volume.
Next, the back-chamber volume (feedback chamber volume) in contrast to the front-chamber volume represents the volume of the inner space formed at least among the diaphragm and the substrate 510, that is, the sealed volume after the sound passes through the diaphragm. A larger back chamber corresponds to a higher sensitivity because the disturbing force of the air exerted on the diaphragm from the back chamber gets smaller when the back-chamber volume gets larger, so that the sensed signal is not distorted. Thus, it is preferred to provide the larger back-chamber volume for the microphone packaging. In addition, it is to be particularly noted that the space inside the back chamber must be completely sealed (only the kept diaphragm is connected to the outside). However, such conventional package is large in form factor, which cannot meet the shrinkage requirement from the development trend of electronics devices (like thinner mobile phone body). Currently, that conventional package thickness is mainly limited by the thicknesses of the substrate and the covering lid. Furthermore, this conventional packaging delivers larger X/Y dimension, which is also facing more and more shrinkage requirements. Another key disadvantage is that the non-monolithic architecture has the worse background noise due to the larger parasitic effect coming from the larger pad area for wire bonding. All these issues will be solved by this invention described hereinafter.